London: At least five former pupils, including two brothers, from a London-based school are believed to have been killed while fighting in Syria and Iraq with the dreaded Islamic State terrorists.

The school, nicknamed "the socialist Eton" after some of the country's prominent Left-leaning politicians sent their children there, is feared to have still some students on the Islamic State (IS) frontline.

A teenager from Holland Park School in west London continues to fight on the front line with IS terrorists and another has been jailed for funding terrorism, The Sunday Times reported.

Britain's security chiefs believe that about 50 people from the UK have died as fighters in the conflict.

One of the dead Britons from the school had led suicide car bombers that allowed IS to seize a key stronghold in Iraq, the report said.

Fatlum Shalaku, 20, was one of the former pupils killed in strikes on government buildings in Ramadi.

Shalaku's older brother Flamur, 23, is said to have also died on the battlefield in Iraq in March.

The brothers, Britons of Kosovan-Albanian descent, travelled to Syria in spring 2013 after becoming radicalised, according to their friends.

They had told their parents they were going to the region to carry out aid work.

Both were close friends of Mohammed Nasser, 21, a third former pupil at Holland Park who was killed fighting for IS last year.

The school, opened in 1958, was famous for its comprehensive education system.

Although it has been given an "outstanding" rating by the UK school regulator Ofsted, the institution now risks being associated with Islamist extremism.